1. Technical Field
The present invention relates to a position calculating method and a position calculating device.
2. Related Art
A GPS (global positioning system) is widely known as a positioning system using a positioning signal and is used for a position calculating device built into a mobile phone, a car navigation apparatus, or the like. The GPS performs position calculation processing for acquiring the three-dimensional coordinate values, which indicate the position of the position calculating device, and a time error on the basis of the information, such as the positions of a plurality of GPS satellites or a pseudo distance from each GPS satellite to the position calculating device.
Known examples of the position calculation processing include position calculation processing using a least squared (LS) method and position calculation processing using a Kalman filter (hereinafter, referred to as a “KF”). In addition, as an application technique of the position calculation processing using the KF, a position calculation processing using an interactive mixing model (hereinafter, referred to as “IMM”) is known in which a plurality of KF models are prepared, position calculation processing is performed for each of the KF models, and these processing results are mixed to determine the final output position (for example, see Yaakov Bar-Shalom, X. Rong Li, Thiagalingam Kirubarajan, “Estimation with Applications to Tracking and Navigation”, (U.S.A.), JOHN WILEY & SONS INC, 2001, p. 92-93, 453-457).
In the IMM, in order to meet various movement states of a position calculating device, it can be considered that the calculation is performed under the conditions in which a KF model corresponding to each assumed movement state is defined beforehand. For example, it is possible to assume a stopped state, a uniform-speed state, and an accelerated/decelerated state as the movement states of a position calculating device and to perform the position calculation processing based on the IMM using a fixed point KF model, a uniform-speed KF model, and a constant acceleration KF model.
In the IMM, the likelihood of the amount of external observations input to the KF model is calculated using a probability density function according to normal distribution (Gaussian distribution). In addition, using the calculated likelihood, a model probability used as the weighting when mixing the processing results of the KF models is calculated. In addition, the output position is determined by mixing the processing results of the KF models using the calculated model probability.
As the amount of external observations input to the KF model, for example, the information on a received frequency or phase of a positioning signal may be used. However, if the receiving environment of the positioning signal is not good, there is a decline in the accuracy of the received frequency or phase of the observed positioning signal. In this case, since the likelihood is not calculated appropriately, the model probability may be calculated incorrectly.
For example, the model probability of the fixed point KF model increases even though the position calculating device is moving. This may result in “position delay” in which the output position is temporally delayed from the actual position. On the contrary, the model probability of the constant acceleration KF model increases even though the position calculating device has stopped. This may result in “position jump” in which the output position significantly deviates from the actual position. These are causes of a lowering of the accuracy of position calculation.